SOFC or SOEC stacks require effective high temperature gaskets to operate efficiently. Such seals must be able to substantially prevent fuel, for example hydrogen, leakage; fuel and oxidant mixing; and oxidant leakage. It is understood that seals should also have similar coefficients of thermal expansion to the surrounding components to avoid stresses. The seals also need to be chemically compatible with the stack components and gases. Furthermore, some seals need to be electrically insulating.
Traditionally, SOFC stack gaskets have been either bonding gaskets (e.g. glass/glass-ceramic or brazes) or non-bonding (compressible) gaskets (For example, see “A review of sealing technologies applicable to solid oxide electrolysis cells” P. Lessing, Journal of Materials Science, 2007, 42 (10), 3465-3476).
The bonding gaskets contain primarily glass and glass-ceramics and they operate by mechanically and chemically bonding to the relevant mating surfaces of the fuel cell. The glass seals are designed to soften and viscously flow above the SOFC operating temperature to provide hermetic sealing. When the SOFC is cooled back down to the operating temperature the glass seals solidify to form a rigid, bonded seal. The drawback of these types of gasket is that they are sensitive to thermo-mechanical stresses, especially in thermal cycling. Changes in thermal expansion coefficients of glasses or glass-ceramics during long term operation can also create additional thermo-mechanical stresses leading to gasket failure.
Non-bonding compressible gaskets are more resistant to thermal cycling as they are not rigidly bonded to adjacent components. However, the leak rates of these types of gaskets are usually higher. The leakage is dominated by interfacial leak paths, especially at low compression stresses. Compressible gaskets also require much higher compressive stresses compared to bonding seals.
More recently, there has been the development of providing multiple material gaskets. The gaskets combine properties from both compressible gaskets and glass-ceramic gaskets. US2003/0203267 A1 discloses a multilayered gasket comprising a mica gasket between outer layers, such as glass or glass-ceramic material.
WO2005/024280 A1 discloses mica seals infiltrated with a glass forming material. WO2009/155184 A1 discloses a double seal having a portion of mica material arranged in proximity to a portion of hermetic sealing material.
Compressive stress is needed in SOFC stacks to ensure adequate sealing performance and to establish good electrical contact between cells and interconnects. The trend in the art is towards larger stacks, meaning a higher compressive force is required and therefore bulkier compression systems. This can lead to higher heat losses and restricted implementation due to design challenges.
Maintaining or improving the sealing properties of the fuel cell gaskets at lower surface stresses is desirable. Lower surface stresses would reduce the size of the compressive systems and result in more degrees of freedom in stack design. For example, thin interconnecting plates would permit more advanced flow geometries and also impart less stress on the relatively fragile cell. This would lead to the creation of more efficient stacks and potential application in more low stress areas.
It is further desirable to provide a gasket that can operate at lower surface stresses whilst maintaining or improving sealing properties over a series of thermal cycles and/or after prolonged use. There is still a further requirement for a gasket to give good properties when a fuel cell has differing pressures between the anode and the cathode. Furthermore, the gasket should provide suitable sealing properties at the desirable operating temperature. It would be advantageous to have the capability of improved sealing even at lower temperatures to reduce long term degradation of the stack.
It is therefore an object of aspects of the present invention to provide a gasket for fuel cells that provides improved properties.
According to a first aspect of the present invention there is provided a gasket for sealing two mating surfaces of a fuel cell comprising a core layer comprising exfoliated vermiculite, said core layer interposed between a first and second coating layer, the said coating layers each comprising glass, glass-ceramic and/or ceramic material.